import contextlib
import copy
from decimal import Decimal

from django.apps.registry import Apps
from django.db.backends.base.schema import BaseDatabaseSchemaEditor
from django.db.backends.ddl_references import Statement
from django.db.transaction import atomic
from django.db.utils import NotSupportedError


class DatabaseSchemaEditor(BaseDatabaseSchemaEditor):

    sql_delete_table = "DROP TABLE %(table)s"
    sql_create_fk = None
    sql_create_inline_fk = "REFERENCES %(to_table)s (%(to_column)s) DEFERRABLE INITIALLY DEFERRED"
    sql_create_unique = "CREATE UNIQUE INDEX %(name)s ON %(table)s (%(columns)s)"
    sql_delete_unique = "DROP INDEX %(name)s"

    def __enter__(self):
        # Some SQLite schema alterations need foreign key constraints to be
        # disabled. Enforce it here for the duration of the schema edition.
        if not self.connection.disable_constraint_checking():
            raise NotSupportedError(
                'SQLite schema editor cannot be used while foreign key '
                'constraint checks are enabled. Make sure to disable them '
                'before entering a transaction.atomic() context because '
                'SQLite3 does not support disabling them in the middle of '
                'a multi-statement transaction.'
            )
        self.connection.cursor().execute('PRAGMA legacy_alter_table = ON')
        return super().__enter__()

    def __exit__(self, exc_type, exc_value, traceback):
        super().__exit__(exc_type, exc_value, traceback)
        self.connection.cursor().execute('PRAGMA legacy_alter_table = OFF')
        self.connection.enable_constraint_checking()

    def quote_value(self, value):
        # The backend "mostly works" without this function and there are use
        # cases for compiling Python without the sqlite3 libraries (e.g.
        # security hardening).
        try:
            import sqlite3
            value = sqlite3.adapt(value)
        except ImportError:
            pass
        except sqlite3.ProgrammingError:
            pass
        # Manual emulation of SQLite parameter quoting
        if isinstance(value, bool):
            return str(int(value))
        elif isinstance(value, (Decimal, float, int)):
            return str(value)
        elif isinstance(value, str):
            return "'%s'" % value.replace("\'", "\'\'")
        elif value is None:
            return "NULL"
        elif isinstance(value, (bytes, bytearray, memoryview)):
            # Bytes are only allowed for BLOB fields, encoded as string
            # literals containing hexadecimal data and preceded by a single "X"
            # character.
            return "X'%s'" % value.hex()
        else:
            raise ValueError("Cannot quote parameter value %r of type %s" % (value, type(value)))

    def _is_referenced_by_fk_constraint(self, table_name, column_name=None, ignore_self=False):
        """
        Return whether or not the provided table name is referenced by another
        one. If `column_name` is specified, only references pointing to that
        column are considered. If `ignore_self` is True, self-referential
        constraints are ignored.
        """
        with self.connection.cursor() as cursor:
            for other_table in self.connection.introspection.get_table_list(cursor):
                if ignore_self and other_table.name == table_name:
                    continue
                constraints = self.connection.introspection._get_foreign_key_constraints(cursor, other_table.name)
                for constraint in constraints.values():
                    constraint_table, constraint_column = constraint['foreign_key']
                    if (constraint_table == table_name and
                            (column_name is None or constraint_column == column_name)):
                        return True
        return False

    def alter_db_table(self, model, old_db_table, new_db_table, disable_constraints=True):
        if disable_constraints and self._is_referenced_by_fk_constraint(old_db_table):
            if self.connection.in_atomic_block:
                raise NotSupportedError((
                    'Renaming the %r table while in a transaction is not '
                    'supported on SQLite because it would break referential '
                    'integrity. Try adding `atomic = False` to the Migration class.'
                ) % old_db_table)
            self.connection.enable_constraint_checking()
            super().alter_db_table(model, old_db_table, new_db_table)
            self.connection.disable_constraint_checking()
        else:
            super().alter_db_table(model, old_db_table, new_db_table)

    def alter_field(self, model, old_field, new_field, strict=False):
        old_field_name = old_field.name
        table_name = model._meta.db_table
        _, old_column_name = old_field.get_attname_column()
        if (new_field.name != old_field_name and
                self._is_referenced_by_fk_constraint(table_name, old_column_name, ignore_self=True)):
            if self.connection.in_atomic_block:
                raise NotSupportedError((
                    'Renaming the %r.%r column while in a transaction is not '
                    'supported on SQLite because it would break referential '
                    'integrity. Try adding `atomic = False` to the Migration class.'
                ) % (model._meta.db_table, old_field_name))
            with atomic(self.connection.alias):
                super().alter_field(model, old_field, new_field, strict=strict)
                # Follow SQLite's documented procedure for performing changes
                # that don't affect the on-disk content.
                # https://sqlite.org/lang_altertable.html#otheralter
                with self.connection.cursor() as cursor:
                    schema_version = cursor.execute('PRAGMA schema_version').fetchone()[0]
                    cursor.execute('PRAGMA writable_schema = 1')
                    references_template = ' REFERENCES "%s" ("%%s") ' % table_name
                    new_column_name = new_field.get_attname_column()[1]
                    search = references_template % old_column_name
                    replacement = references_template % new_column_name
                    cursor.execute('UPDATE sqlite_master SET sql = replace(sql, %s, %s)', (search, replacement))
                    cursor.execute('PRAGMA schema_version = %d' % (schema_version + 1))
                    cursor.execute('PRAGMA writable_schema = 0')
                    # The integrity check will raise an exception and rollback
                    # the transaction if the sqlite_master updates corrupt the
                    # database.
                    cursor.execute('PRAGMA integrity_check')
            # Perform a VACUUM to refresh the database representation from
            # the sqlite_master table.
            with self.connection.cursor() as cursor:
                cursor.execute('VACUUM')
        else:
            super().alter_field(model, old_field, new_field, strict=strict)

    def _remake_table(self, model, create_field=None, delete_field=None, alter_field=None):
        """
        Shortcut to transform a model from old_model into new_model

        The essential steps are:
          1. rename the model's existing table, e.g. "app_model" to "app_model__old"
          2. create a table with the updated definition called "app_model"
          3. copy the data from the old renamed table to the new table
          4. delete the "app_model__old" table
        """
        # Self-referential fields must be recreated rather than copied from
        # the old model to ensure their remote_field.field_name doesn't refer
        # to an altered field.
        def is_self_referential(f):
            return f.is_relation and f.remote_field.model is model
        # Work out the new fields dict / mapping
        body = {
            f.name: f.clone() if is_self_referential(f) else f
            for f in model._meta.local_concrete_fields
        }
        # Since mapping might mix column names and default values,
        # its values must be already quoted.
        mapping = {f.column: self.quote_name(f.column) for f in model._meta.local_concrete_fields}
        # This maps field names (not columns) for things like unique_together
        rename_mapping = {}
        # If any of the new or altered fields is introducing a new PK,
        # remove the old one
        restore_pk_field = None
        if getattr(create_field, 'primary_key', False) or (
                alter_field and getattr(alter_field[1], 'primary_key', False)):
            for name, field in list(body.items()):
                if field.primary_key:
                    field.primary_key = False
                    restore_pk_field = field
                    if field.auto_created:
                        del body[name]
                        del mapping[field.column]
        # Add in any created fields
        if create_field:
            body[create_field.name] = create_field
            # Choose a default and insert it into the copy map
            if not create_field.many_to_many and create_field.concrete:
                mapping[create_field.column] = self.quote_value(
                    self.effective_default(create_field)
                )
        # Add in any altered fields
        if alter_field:
            old_field, new_field = alter_field
            body.pop(old_field.name, None)
            mapping.pop(old_field.column, None)
            body[new_field.name] = new_field
            if old_field.null and not new_field.null:
                case_sql = "coalesce(%(col)s, %(default)s)" % {
                    'col': self.quote_name(old_field.column),
                    'default': self.quote_value(self.effective_default(new_field))
                }
                mapping[new_field.column] = case_sql
            else:
                mapping[new_field.column] = self.quote_name(old_field.column)
            rename_mapping[old_field.name] = new_field.name
        # Remove any deleted fields
        if delete_field:
            del body[delete_field.name]
            del mapping[delete_field.column]
            # Remove any implicit M2M tables
            if delete_field.many_to_many and delete_field.remote_field.through._meta.auto_created:
                return self.delete_model(delete_field.remote_field.through)
        # Work inside a new app registry
        apps = Apps()

        # Provide isolated instances of the fields to the new model body so
        # that the existing model's internals aren't interfered with when
        # the dummy model is constructed.
        body = copy.deepcopy(body)

        # Work out the new value of unique_together, taking renames into
        # account
        unique_together = [
            [rename_mapping.get(n, n) for n in unique]
            for unique in model._meta.unique_together
        ]

        # Work out the new value for index_together, taking renames into
        # account
        index_together = [
            [rename_mapping.get(n, n) for n in index]
            for index in model._meta.index_together
        ]

        indexes = model._meta.indexes
        if delete_field:
            indexes = [
                index for index in indexes
                if delete_field.name not in index.fields
            ]

        # Construct a new model for the new state
        meta_contents = {
            'app_label': model._meta.app_label,
            'db_table': model._meta.db_table,
            'unique_together': unique_together,
            'index_together': index_together,
            'indexes': indexes,
            'apps': apps,
        }
        meta = type("Meta", (), meta_contents)
        body['Meta'] = meta
        body['__module__'] = model.__module__

        temp_model = type(model._meta.object_name, model.__bases__, body)

        # We need to modify model._meta.db_table, but everything explodes
        # if the change isn't reversed before the end of this method. This
        # context manager helps us avoid that situation.
        @contextlib.contextmanager
        def altered_table_name(model, temporary_table_name):
            original_table_name = model._meta.db_table
            model._meta.db_table = temporary_table_name
            yield
            model._meta.db_table = original_table_name

        with altered_table_name(model, model._meta.db_table + "__old"):
            # Rename the old table to make way for the new
            self.alter_db_table(
                model, temp_model._meta.db_table, model._meta.db_table,
                disable_constraints=False,
            )
            # Create a new table with the updated schema.
            self.create_model(temp_model)

            # Copy data from the old table into the new table
            field_maps = list(mapping.items())
            self.execute("INSERT INTO %s (%s) SELECT %s FROM %s" % (
                self.quote_name(temp_model._meta.db_table),
                ', '.join(self.quote_name(x) for x, y in field_maps),
                ', '.join(y for x, y in field_maps),
                self.quote_name(model._meta.db_table),
            ))

            # Delete the old table
            self.delete_model(model, handle_autom2m=False)

        # Run deferred SQL on correct table
        for sql in self.deferred_sql:
            self.execute(sql)
        self.deferred_sql = []
        # Fix any PK-removed field
        if restore_pk_field:
            restore_pk_field.primary_key = True

    def delete_model(self, model, handle_autom2m=True):
        if handle_autom2m:
            super().delete_model(model)
        else:
            # Delete the table (and only that)
            self.execute(self.sql_delete_table % {
                "table": self.quote_name(model._meta.db_table),
            })
            # Remove all deferred statements referencing the deleted table.
            for sql in list(self.deferred_sql):
                if isinstance(sql, Statement) and sql.references_table(model._meta.db_table):
                    self.deferred_sql.remove(sql)

    def add_field(self, model, field):
        """
        Create a field on a model. Usually involves adding a column, but may
        involve adding a table instead (for M2M fields).
        """
        # Special-case implicit M2M tables
        if field.many_to_many and field.remote_field.through._meta.auto_created:
            return self.create_model(field.remote_field.through)
        self._remake_table(model, create_field=field)

    def remove_field(self, model, field):
        """
        Remove a field from a model. Usually involves deleting a column,
        but for M2Ms may involve deleting a table.
        """
        # M2M fields are a special case
        if field.many_to_many:
            # For implicit M2M tables, delete the auto-created table
            if field.remote_field.through._meta.auto_created:
                self.delete_model(field.remote_field.through)
            # For explicit "through" M2M fields, do nothing
        # For everything else, remake.
        else:
            # It might not actually have a column behind it
            if field.db_parameters(connection=self.connection)['type'] is None:
                return
            self._remake_table(model, delete_field=field)

    def _alter_field(self, model, old_field, new_field, old_type, new_type,
                     old_db_params, new_db_params, strict=False):
        """Perform a "physical" (non-ManyToMany) field update."""
        # Alter by remaking table
        self._remake_table(model, alter_field=(old_field, new_field))
        # Rebuild tables with FKs pointing to this field if the PK type changed.
        if old_field.primary_key and new_field.primary_key and old_type != new_type:
            for rel in new_field.model._meta.related_objects:
                if not rel.many_to_many:
                    self._remake_table(rel.related_model)

    def _alter_many_to_many(self, model, old_field, new_field, strict):
        """Alter M2Ms to repoint their to= endpoints."""
        if old_field.remote_field.through._meta.db_table == new_field.remote_field.through._meta.db_table:
            # The field name didn't change, but some options did; we have to propagate this altering.
            self._remake_table(
                old_field.remote_field.through,
                alter_field=(
                    # We need the field that points to the target model, so we can tell alter_field to change it -
                    # this is m2m_reverse_field_name() (as opposed to m2m_field_name, which points to our model)
                    old_field.remote_field.through._meta.get_field(old_field.m2m_reverse_field_name()),
                    new_field.remote_field.through._meta.get_field(new_field.m2m_reverse_field_name()),
                ),
            )
            return

        # Make a new through table
        self.create_model(new_field.remote_field.through)
        # Copy the data across
        self.execute("INSERT INTO %s (%s) SELECT %s FROM %s" % (
            self.quote_name(new_field.remote_field.through._meta.db_table),
            ', '.join([
                "id",
                new_field.m2m_column_name(),
                new_field.m2m_reverse_name(),
            ]),
            ', '.join([
                "id",
                old_field.m2m_column_name(),
                old_field.m2m_reverse_name(),
            ]),
            self.quote_name(old_field.remote_field.through._meta.db_table),
        ))
        # Delete the old through table
        self.delete_model(old_field.remote_field.through)
